1. Field of the Invention
The present invention relates to a method for fabricating a polycrystalline silicon thin film for an apparatus and an apparatus that uses polycrystalline silicon thin film fabricated by the method, more particularly, to a method for fabricating a polycrystalline silicon thin film capable of controlling the shape of grains of polycrystalline silicon thin film and an apparatus using the polycrystalline silicon thin film.
2. Discussion of Related Art
Generally, the sequential lateral solidification (SLS) crystallizing method is used to laterally grow grain silicon by irradiating a laser beam on an amorphous silicon layer two or more times. Polycrystalline silicon grains thus fabricated are formed in a columnar shape, and grain boundaries are formed between adjacent grains due to the grains' limited size.
Polycrystalline or single crystal large silicon grains may be formed on a substrate using SLS crystallization technology, and characteristics similar to characteristics of a thin film transistor (TFT) fabricated of single crystal silicon may be obtained.
FIG. 1A, FIG. 1B and FIG. 1C show an ordinary SLS crystallizing method.
In the SLS crystallizing method as illustrated in FIG. 1A, a laser beam is irradiated onto an amorphous silicon thin film layer through a mask having a laser beam transmission region and a laser beam non-transmission region, thereby melting the amorphous silicon in the laser beam transmission region.
Crystallization preferentially occurs at an interface between amorphous silicon and molten silicon if cooling is started after finishing the laser beam irradiation, wherein a temperature gradient is formed in which temperature is gradually decreased in a direction from the interface between amorphous silicon and molten silicon to a molten silicon layer.
Therefore, referring to FIG. 1B, a polycrystalline silicon thin film layer, with laterally grown grains formed in a columnar shape, is formed since heat flux flows in a direction from the interface of the mask to a central part of the molten silicon layer. The polycrystalline silicon grains grow laterally until the molten silicon layer is completely solidified.
As illustrated in FIG. 1C, amorphous silicon and crystalline silicon are melted by moving stage, thereby moving mask, and by irradiating a laser beam onto partially exposed portions of the amorphous silicon thin film layer and already crystallized polycrystalline silicon layer. Silicon atoms are adhered to already formed polycrystalline silicon grains that are covered by the mask so that the length of the grains is increased as the melted amorphous silicon and crystalline silicon cool after being melted.
FIG. 2A, FIG. 2B and FIG. 2C are plan figures that show a method for crystallizing grain silicon using a mask structure of an ordinary fabrication method of polycrystalline silicon thin film, and FIG. 3A, FIG. 3B and FIG. 3C are plan figures of polycrystalline silicon thin films produced in respective stages.
In FIG. 2A, amorphous silicon is melted by irradiating a laser beam onto the amorphous silicon using an ordinary mask with a laser beam transmission region and a laser beam non-transmission region. Polycrystalline silicon is formed as the melted amorphous silicon solidifies.
The mask is shifted as far as a certain distance illustrated in FIG. 2B, and a laser beam is irradiated onto a portion of the previously formed polycrystalline silicon and amorphous silicon as illustrated in FIG. 2C. By continuously scanning the polycrystalline silicon and irradiating a laser beam onto the polycrystalline silicon in this manner, at a part where mask patterns of the amorphous silicon and transmission region overlap with each other, polycrystalline silicon is melted and crystallized as it solidifies.
Polycrystalline silicon crystallinity varies per parts onto which a laser pulse is irradiated due to laser shot energy deviation, or energy density in the laser beam can be uneven in amorphous silicon onto which a laser beam is once irradiated as illustrated in FIG. 3A.
Particularly, laser scan line causes striped defects on upper and lower boundaries between different laser shots as illustrated in FIG. 3B and FIG. 3C.
These striped defects cause luminance non-uniformity on a display device, particularly an organic electroluminescent device.
PCT international patent No. WO 97/45827 and U.S. Pat. No. 6,322,625 disclose technologies for converting amorphous silicon on a substrate into polycrystalline silicon, or for crystallizing only a selected region on the substrate, by the sequential lateral solidification (SLS) method.
Additionally, obtaining TFT characteristics second only to single crystal silicon is disclosed in U.S. Pat. No. 6,177,391, since the barrier effect of grain boundaries for a carrier direction is minimized when an active channel direction is parallel to a direction of grains grown by SLS crystallizing method. But the patent also discloses that large numbers of grain boundaries act as a trap of charge carriers, and TFT characteristics greatly deteriorate when the active channel direction is perpendicular to the grain growing direction.
There is a case, however, where an active matrix display device is fabricated with driving circuit TFTs generally perpendicular to pixel cell region TFTs, wherein uniformity of the display device is improved when an active channel region direction is inclined 30 to 60 degrees to a crystal growing direction.
However, with this method, since the grains are formed by the SLS crystallizing method, the problem of non-uniform grains due to non-uniformity of laser energy density still exists.
Also, with this method, crystallization can not be carried out all over a substrate, therefore an uncrystallized region will always exist, although a method is described in Korean Patent Laid-open Publication No. 2002-93194 in which laser beam patterns are formed in a triangle shape (“”), and crystallization is proceeded as moving the triangle shaped (“”) laser beam patterns widthwise.